Process for producing lithium manganate and lithium battery using the lithium manganate

ABSTRACT

According to the process of this invention, first a manganese oxide seed is prepared, which is then grown to obtain manganese oxide having large particle diameters. The manganese oxide thus obtained is reacted with a lithium compound, whereby lithium manganate having large particle diameters can be obtained. Since the lithium manganate has large particle diameters and gives a high packing density, lithium batteries with a high energy density can be provided by using the lithium manganate.

TECHNICAL FIELD

[0001] This invention relates to a process for producing lithiummanganate, which is a compound useful as the positive electrode materialof a lithium battery, and to a lithium battery obtained by using thelithium manganate produced by the process.

BACKGROUND ART

[0002] Lithium secondary batteries have rapidly come into wide use inrecent years because they are of high voltage, excellent in charge anddischarge characteristics, moreover light in weight and small in size,particularly those having a high electromotive force of 4-V class beingstrongly demanded. As to such lithium secondary batteries, there areknown those which use a complex oxide of cobalt or nickel with lithiumas the positive electrode active material; but cobalt and nickel areexpensive and moreover have the problem of possible exhaustion ofresources in the future.

[0003] Lithium manganate, which is a complex oxide of manganese andlithium, represented by the chemical formula LiMn₂O₄ and has a spineltype crystal structure, is useful as the positive electrode activematerial of 4-V class lithium secondary batteries. Furthermore, sincemanganese of the raw material is inexpensive and is rich in resources,lithium manganate is promising as a material which can replace lithiumcobaltate and lithium nickelate.

[0004] Positive electrode active materials are kneaded with variousadditives and then shaped, or additionally incorporated with a solventto form a paste and then coated on a substrate. Lithium manganateobtained by the conventional wet process is only of small particlediameters and, even when it is subjected to fireing to effect particlegrowth, desired large particles cannot be obtained. Resultantly, itshows a low packing density and cannot be filled in a large amount in afixed volume, so that it cannot give a product of high energy density.It is generally considered that the packing density of powder increasesas its particle diameter is increased (that is, its specific surfacearea is decreased); accordingly lithium manganate with a large particlediameter is greatly demanded.

[0005] JP-A-10-194745 discloses a method of increasing the particlediameter of lithium manganate which comprises mixing a manganese oxideand a lithium salt, and subjecting the mixture to primary fireing, thento a treatment for decreasing crystallinity, e.g., mechanical grinding,and further to secondary fireing. In this method, however, since thereactivity of a manganese compound with a lithium compound is poor, ahomogeneous composition can be hardly obtained even when fireing isconducted at high temperatures, and lithium manganate having manylattice defects is produced. Moreover, since the product is anon-uniform sintered body obtained by sintering particles, the particlediameter and the particle shape are difficult to control.

[0006] JP-A-10-172567 discloses a method which comprises mixingmanganese dioxide or a manganese compound with a lithium compound in anaqueous solution, then drying the mixture with a spray drier,granulating the dried product, followed by fireing JP-A-10-297924discloses a method which comprises synthesizing lithium manganatepowder, then densifying and agglomerating the powder, followed byclassification and granulation, and then fireing the granulated product.Though these methods give lithium manganate, which makes the basesubstance, that has few lattice defects and has a uniform composition,difficulties remain unsolved in that the particle diameter and particleshape are difficult to control and that the particles ultimatelyobtained are non-uniform sintered bodies.

DISCLOSURE OF THE INVENTION

[0007] The object of this invention is to provide, overcoming thedifficulties of the prior art described above, a process for producingwith industrial and economical advantage lithium manganate suitable forhigh energy density lithium batteries which has a large particlediameter and shows a high packing density, has a uniform particlediameter and particle shape, and further has few lattice defects and auniform composition.

[0008] After extensive study the present inventors have found that whena manganese compound and a basic compound are reacted with each other ina solution and oxidized to obtain manganese oxide seed, then a manganesecompound and a basic compound are reacted with each other in a solutionin which the manganese oxide seed is present, the reaction product isoxidized and made to grow to a desired large particle diameter, and theproduct thus obtained is used for synthesizing lithium manganate,lithium manganate which has a large particle diameter and has a uniformparticle diameter distribution and uniform particle shape can beproduced, and further that when the above-mentioned manganese oxide anda lithium compound are reacted with each other in a solution, oralternatively a part of manganese of the manganese oxide is replaced bya proton to improve reactivity and the resulting modified manganeseoxide is mixed with a lithium compound or reacted therewith in asolution, and the product obtained by either of the reactions is heatedand fired, lithium manganate having an excellent crystallinity and largeparticle diameters can be obtained.

[0009] Thus, according to this invention, there are provided a processfor producing lithium manganate which comprises (1) the step of reactinga manganese compound with a basic compound to obtain manganesehydroxide, (2) the step of oxidizing the manganese hydroxide to obtainmanganese oxide seed, (3) the step of conducting oxidation whilereacting a manganese compound with a basic compound in the presence ofthe manganese oxide seed to cause the particle growth of the manganeseoxide seed, (4) the step of reacting the manganese oxide which hasundergone particle growth with a lithium compound, or the step oftreating the manganese oxide with an acid to obtain proton-substitutedmanganese oxide and then mixing the proton-substituted manganese oxidewith a lithium compound or reacting it with a lithium compound, and (5)the step of fireing the reaction product or the mixture obtained abovewith heating, and a lithium battery which uses the lithium manganateobtained by the process as the positive electrode active material.

BEST MODE FOR CARRYING OUT THE INVENTION

[0010] In this invention, as the first step, manganese hydroxide isformed by reacting a manganese compound with a basic compound in aliquid medium such as water. To facilitate the control of the oxidationlevel in the next, second step, the reaction of forming the manganesehydroxide is preferably conducted in an inert gas atmosphere. The use ofnitrogen gas as the inert gas is advantageous both industrially andeconomically. The manganese compound used in the first step and thethird step described later may be either a water-soluble manganesecompound or a water-unsoluble manganese compound made into an acidsolution, but water-soluble one is more preferable. The water-solublemanganese compounds used may be inorganic manganese compounds, such asmanganese sulfate, manganese chloride and manganese nitrate, and organicmanganese compounds, such as manganese acetate, inorganic manganesecompounds being preferred. The basic compound used may be alkali metalhydroxides, such as sodium hydroxide, potassium hydroxide and lithiumhydroxide and ammonium compounds, such as ammonia gas and ammoniaaqueous solution.

[0011] In the first step, the manganese compound may be whollyneutralized with an equimolar or more amount of basic compound, or itmay be partially neutralized with a less than equimolar amount of basiccompound. Partial neutralization is more preferable because manganesehydroxide formed by partial neutralization of a manganese compound willhave an increased particle diameter. When partial neutralization isconducted such that the concentration of manganese ions remaining in theliquid medium after partial neutralization is 5-60 g/l, preferably 10-40g/l, products having a particularly large particle diameter can beobtained.

[0012] In the next, second step, the manganese hydroxide obtained in thefirst step is oxidized to form manganese oxide seed. As describedbefore, the particle diameter of the manganese oxide seed can becontrolled by appropriately setting the concentration of manganese ionswhich remain in the solution after partial neutralization. When theremaining manganese ion concentration is adjusted to the above-mentionedrange of 5-60 g/l, the average particle diameter of the seed comes to be0.1-0.4 μm; particularly when the remaining manganese ion concentrationis controlled in the range of 10-40 g/l, the particle diameter of theseed reaches the maximum, and the average particle diameter of 0.3-0.4μm is attained. Though the oxidation may be conducted in a gas phaseafter the manganese hydroxide formed has been collected by filtrationand washed, it is more advantageously conducted in a liquid phase byblowing an oxidizing gas, such as air, oxygen and ozone, into the liquidor by adding an oxidizing agent, such as aqueous hydrogen peroxide andperoxodisulfates, because then the oxidation can be conducted insuccession to the first step.

[0013] In the third step, while, in a solution, e.g. an aqueoussolution, containing the manganese oxide seed obtained in the secondstep, a newly added manganese compound or the remaining manganesecompound is being reacted with a newly added basic compound, oxidationis conducted as described above by blowing an oxidizing gas into thereaction system or adding an oxidizing agent thereto, thereby to effectthe growth of seed particles. In particular, the use of an oxidizing gasis preferable and the use of air as the oxidizing gas is morepreferable. Since the growth of particles can be controlled by properlysetting reaction conditions, such as the amount of the basic compoundadded and the method of addition, manganese oxide having a uniformparticle size distribution and uniform particle shape can be obtained.Though the oxidation degree of the manganese oxide may be set asdesired, it is preferably oxidized to a state represented by the formulaMn₃O₄ or 2MnO.MnO₂.

[0014] The manganese compound and the basic compound used are asdescribed for the first step.

[0015] Also in the third step, the reaction of the manganese compoundwith the basic compound may be conducted in the manner of either wholeamount neutralization or partial neutralization. Method of whole amountneutralization is industrially more advantageous because virtually nounreacted manganese compound remains behind. In partial neutralization,manganese oxide having fine particle diameters is hardly formed, so thatthe method is advantageous for obtaining a product having a uniformshape and particle size distribution. When partial neutralization isconducted for example, the concentration of manganese ions remainingafter the partial neutralization in the liquid medium is preferablycontrolled to be in the range of 5-60 g/l, more preferably 10-40 g/l;the solution of the manganese compound and the solution of the basiccompound are preferably added in parallel because more uniform particlesare formed thereby.

[0016] In the fourth step, the manganese oxide which has undergoneparticle growth and a lithium compound are reacted with each other in aliquid medium, such as water, or the manganese compound which hasundergone particle growth is treated with an acid to obtainproton-substituted manganese oxide, which is then reacted with a lithiumcompound in a liquid medium, such as water; thus a lithium manganateprecursor is formed. Alternatively, the proton-substituted manganesecompound is mixed with a lithium compound in a solid phase to form amixture. It is preferable that, in advance to the reaction with alithium compound, the manganese oxide is treated beforehand with an acidand converted into proton-substituted manganese oxide, because thereactivity with the lithium compound is improved thereby. Though theproton-substituted manganese oxide may be mixed with a lithium compoundeither in a liquid phase or, after having been collected by filtration,in a solid phase, it is more preferable to react the two compounds witheach other in a liquid medium, such as water, to form a lithiummanganate precursor. The term “lithium manganate precursor” refers notto a mere mixture of a manganese oxide and a lithium compound but to asubstance wherein lithium ions have been incorporated into the crystalstructure of manganese oxide as described later.

[0017] The lithium compounds which may be used in the fourth step are,for example, lithium hydroxide, lithium nitrate, lithium carbonate,lithium hydrogen carbonate, lithium chloride and lithium sulfate.Particularly preferred among them are basic compounds, such as lithiumhydroxide, because of their excellent reactivity. The composition ratioof manganese to lithium in the intended lithium manganate may becontrolled by the reaction amounts of the above-mentioned compoundscorresponding thereto. For example, when a proton-substituted manganesecompound is reacted with a lithium compound in an aqueous medium, thereaction amounts of the both compounds can be determined by measuringthe alkali concentration of the reaction liquid sampled in a smallportion.

[0018] The proton-substituted manganese oxide referred to in thisinvention is a substance which is formed by treating manganese oxidewith an acid and in which presumably a part of manganese ions in themanganese oxide have been replaced by hydrogen ions. It is estimatedthat the substituted hydrogen ions are active and are readilyexchangeable with other cations. When the proton-substituted manganeseoxide is reacted with a lithium compound, lithium ions are readilyincorporated into the manganese oxide through exchange reaction withhydrogen ions. Though the acid used in the acid treatment is notparticularly limited and may be inorganic acids, such as hydrochloricacid, sulfuric acid, nitric acid and hydrofluoric acid, and organicacids, such as acetic acid and formic acid, inorganic acids arepreferred because they are highly effective, sulfuric acid orhydrochloric acid being more preferred because the acid treatment can beconducted with industrial advantage. The concentration of the acid addedis preferably 0.05-10N. Acid concentration lower than theabove-mentioned range is industrially disadvantageous because thenecessary amount of the acid to be added tends to be too large and theslurry concentration tends to be too low. Acid concentration higher thanthe range is unfavorable because then the manganese oxide tends todecompose readily.

[0019] When manganese oxide and a lithium compound, orproton-substituted manganese oxide and a lithium compound, are reactedwith each other in a liquid medium such as water, the reaction may beconducted usually in the range of 70-250° C. In this instance, forexample, when the temperature is not higher than 100° C., the reactionmay be conducted under atmospheric pressure and hence does not need theuse of a pressure reaction vessel, which offers a practical advantage;on the other hand, a temperature not lower than 100° C. is advantageousin that the reaction proceeds more readily. The preferred temperaturerange is 80-230° C. and the more preferred range is 85-180° C. In areaction at 100° C. or above, a pressure vessel, such as an autoclave,is used and a hydrothermal treatment may be applied under saturatedsteam pressure or under pressurization. The reaction proceeds readilywhen air, oxygen, ozone, etc. as an oxidizing gas, or aqueous hydrogenperoxide, peroxodisulfates, etc. as an oxidizing agent, are supplied tothe reaction system. Particularly, the use of an oxidizing gas ispreferable and the use of air as the oxidizing gas is more preferable.

[0020] The proton-substituted manganese oxide may also be merely mixedwith a lithium compound and used as such in the next, fifth step.

[0021] In the fifth step, a lithium manganate precursor, or a mixture ofproton-substituted manganese oxide with a lithium compound, is firedwith heating to produce lithium manganate. In this invention, since inthe third step the manganese oxide particles have been grown nearly tothe particle diameter of the intended large-particle lithium manganate,the fireing temperature requires only to be not lower than thetemperature at which the phase change of these materials to lithiummanganate takes place. Though the fireing temperature may vary dependingon the composition and particle size of the above-mentioned precursor orthe mixture, the fireing atmosphere, and other factors, it is generallynot lower than 250° C., preferably not higher than 850° C. to preventsintering and more preferably not lower than 280° C. and not higher than800° C. The fireing atmosphere is not particularly limited so long as itis an oxygen-containing atmosphere, such as air; the oxygen partialpressure may be set as required.

[0022] The lithium manganate obtained by fireing with heating may besubjected, according to necessity, to grinding or crushing, or todensifying. The method for densification is not particularly limited.The densification may be conducted either by compression forming usingan edge runner mill, screen forming machine, extrusion forming machine,pressure roll, mixing mill, etc. or by stirring-granulation using amixer, etc.

[0023] The lithium manganate obtained by the process of this inventionis a compound represented by the formula Li_(X)Mn_(Y)O₄, wherein thevalue of X/Y is preferably in the range of 0.3-1.5. Particularlypreferred are those which have a spinel-type crystal structure and arerepresented by the formulas LiMn₂O₄, Li_(4/3)Mn_(5/3)O₄ or the like. Thelithium manganate may comprise either of a single phase of lithiummanganate or of a mixture of lithium manganate with manganese oxide.

[0024] The lithium manganate of this invention has a uniform particlesize distribution and uniform particle shape, and has a large averageparticle diameter of 0.4-50 μm, preferably 0.8-30 μm, more preferably1-20 μm, so that it shows a high packing density; for example, it showsa bulk density of 1.5-2.2 g/cc as determined by tapping. Accordingly, itcan be filled as a positive electrode active material in a large amountinto moldings or pastes, so that when the resulting electrode is used asthe positive electrode, a lithium battery having a high energy densityis obtained. When the particle diameter is smaller than theabove-mentioned range, an intended packing density cannot be obtained;when it is larger than the range, the lithium battery obtained by usingthe lithium manganate cannot have intended characteristic property. Theaverage particle diameter referred to herein was determined by the laserscattering method and the specific surface area by the BET method.

[0025] Next, this invention provides a lithium battery which uses theabove-mentioned lithium manganate as the positive electrode activematerial. The lithium batteries referred to in this invention is aprimary battery using lithium metal for the negative electrode, achargeable secondary battery using lithium metal for the negativeelectrode and a chargeable lithium ion secondary battery using acarbonaceous material, tin compound, lithium titanate, etc. for thenegative electrode. Since the lithium manganate of this invention hasfew lattice defects and is excellent in crystallinity, when it is usedparticularly as the positive electrode active material of a lithiumsecondary battery, a positive electrode is obtained which hardlyundergoes disintegration of crystals at the time of charge anddischarge, and gives excellent battery characteristics. Further, whenthe lithium manganate of this invention which has a crystal structurecomprising mainly the spinel structure is used, 3-V class lithiumsecondary batteries which can be charged and discharged in a voltageregion of about 2-3.5V and 4-V class ones for which the voltage regionis about 3.5-4.5V can be obtained; such lithium manganate isparticularly useful for 4-V class batteries.

[0026] The positive electrode for lithium batteries, when it is used forcoin type batteries, may be obtained by adding to the lithium manganatepowder of this invention carbonaceous conductive materials, such asacetylene black, carbon and graphite powder, and binders, such aspolytetrafluoroethylene resin and polyvinylidene fluoride, and kneadingand molding the resulting mixture. To be used for cylindrical or squarebatteries, the positive electrode may be obtained by adding to thelithium manganate powder of this invention organic solvents, such asN-methylpyrrolidone, in addition to the above-mentioned additives,kneading the resulting mixture into the form of paste, and coating thepaste on a metallic current collector, such as aluminum foil, followedby drying.

[0027] The electrolyte used for a lithium battery may be a solution oflithium ions dissolved in a polar organic solvent which iselectrochemically stable, that is, neither oxidized nor reduced in avoltage range wider than the range in which it works as a lithium ionbattery. The polar organic solvent used may be, for example, propylenecarbonate, ethylene carbonate, diethyl carbonate, dimethoxyethane,tetrahydrofuran, γ-butyrolactone, and the liquid mixtures thereof. Thesolute which is used as the lithium ion source may be, for example,lithium perchlorate, lithium hexafluorophosphate and lithiumtetrafluoroborate. Porous polypropylene film or polyethylene film isplaced as a separator between the electrodes.

[0028] The batteries include, for example, those of coin type which areobtained by placing a separator between a pellet-formed positiveelectrode and negative electrode, pressure-bonding the resultingassembly to a sealable can provided with a gasket made of polyproylene,pouring an electrolyte into the can, and tightly sealing the can, andthose of cylindrical type obtained by coating the positive electrodematerial and the negative electrode material on a metallic currentcollector, coiling the current collector with a separator heldtherebetween, placing the resulting assembly in a battery can providedwith a gasket, pouring an electrolyte into the can, and sealing the can.Further, there is known a three-electrode type battery intended fordetermining the electrochemical characteristics of a battery. Thisbattery is provided with a reference electrode besides the positiveelectrode and the negative electrode and is used for evaluating theelectrochemical characteristics of the respective electrodes bycontrolling the voltages of other electrodes against the referenceelectrode.

[0029] For evaluation of the performance characteristics of lithiummanganate as a positive electrode material, a secondary battery may beconstructed by using metallic lithium or the like as the negativeelectrode and its capacity can be determined by charging and dischargingat a constant current in a suitable voltage range. Further, by repeatingcharge and discharge, its cycle characteristic property can be judgedfrom the change of the capacity.

[0030] Embodiments

[0031] This invention is described in detail below with reference toExamples, but the invention is in no way limited thereto.

EXAMPLE 1

[0032] 1. Synthesis of Manganese Hydroxide

[0033] In a reaction vessel made of stainless steel were placed 2.57 lof an 8.5 mol/l sodium hydroxide solution and 1.805 l of water. Whilenitrogen gas was being blown thereinto at a rate of 5 l/min, a solutionof 3.744 kg of manganese sulfate (88.06% content in terms of MnSO₄)dissolved in 15 kg of water was rapidly added thereto to effectneutralization, with stirring, at 70° C. and aged for 3 hours to obtainmanganese hydroxide. The concentration of manganese ions remaining inthe solution after the neutralization was 30 g/l.

[0034] 2. Synthesis of Manganese Oxide Seed

[0035] Into the solution containing manganese hydroxide obtained above,with stirring, was blown an air-nitrogen (1:1) gas mixture at a rate of5 l/min to effect oxidation at a temperature of 70° C., and theoxidation was finished at the time when the pH had reached 6.4. Thus, amanganese oxide seed was prepared.

[0036] 3. Growth of Manganese Oxide Seed

[0037] While the solution containing manganese oxide obtained above waskept at 70° C. and stirred, an air-nitrogen (1:1) gas mixture was blownthereinto at a rate of 5 l/min, and an aqueous solution of 3.744 kg ofmanganese sulfate (88.06% content in terms of MnSO₄) dissolved in 16.843kg of water and 2.57 l of an 8.5 mol/l sodium hydroxide solution wereconcurrently added thereto over 16 hours to effect neutralization andoxidation, thus to make the manganese oxide seed grow. When the pH hadreached 6.4, a 20 l portion of the slurry was withdrawn, andsuccessively, while an air-nitrogen (1:1) gas mixture was being blowninto the slurry, an aqueous manganese sulfate solution and a sodiumhydroxide solution were concurrently added thereto according to the sameprocedure as described above, to effect further growth of the manganeseoxide seed. When the pH had reached 6.4 the growth reaction wasfinished, and the solid in the reaction mixture was collected byfiltration and washed with water to obtain manganese oxide. Theconcentration of manganese ions remaining in the solution at thecompletion of reaction was 30 g/l. (Sample a).

[0038] 4. Synthesis of Proton-Substituted Manganese Oxide

[0039] A slurry of manganese oxide (700 g in terms of Mn) dispersed inwater was placed in a stainless steel reaction vessel and warmed to 60°C. Into the slurry was added with stirring 2.039 l of 1 mol/l sulfuricacid over 1 hour; thereafter the resulting mixture was allowed to reactfor 2 hours, then filtered and washed with water to obtainproton-substituted manganese oxide.

[0040] 5. Synthesis of Lithium Manganate Precursor

[0041] To a slurry of the proton-substituted manganese oxide (500 g interms of Mn) dispersed in water was added 5.448 mols of lithiumhydroxide monohydrate and dissolved therein, then water was added to theslurry to make a volume of 1.667 l, and the resulting slurry was placedin a glass reaction vessel. Air was blown into the slurry at a rate of 2l/min and, with stirring, the slurry was heated to 90° C. and allowed toreact for 5 hours. Then the reaction mixture was transferred to anautoclave and subjected to a hydrothermal treatment at 130° C. for 3hours. The hydrothermally treated slurry was again placed in a glassreaction vessel and, while air was being blown thereinto at a rate of 2l/min to effect stirring, was further allowed to react at 90° C. for 1hour. After the reaction, the slurry was cooled to a temperature of 60°C., then filtered and washed with 2 l of a 0.1 mol/l lithium hydroxidesolution to obtain a lithium manganate precursor.

[0042] 6. Fireing of Lithium Manganate Precursor

[0043] The lithium manganate precursor was dried at 110° C. for 12 hoursand then fired in air at 750° C. for 3 hours to obtain lithiummanganate.

[0044] 7. Densification of Lithium Manganate

[0045] Two hundred grams of lithium manganate fired above was subjectedto grinding and densifying for 30 minutes on a small edge runner mill(mfd. by YOSHIDA SEISAKUSYO CO., LTD.). (Sample A)

EXAMPLE 2

[0046] 1. Synthesis of Manganese Hydroxide

[0047] Manganese hydroxide was obtained according to the same method asin Example 2.

[0048] 2. Synthesis of Manganese Oxide Seed

[0049] A manganese oxide seed was obtained according to the same methodas in Example 1.

[0050] 3. Growth of Manganese Oxide Seed

[0051] The solution containing the manganese oxide seed obtained abovewas heated to 70° C. and, with stirring, an air-nitrogen gas mixture(1:1) was blown thereinto at a rate of 5 l/min. At the same time, anaqueous solution of 3.744 kg of manganese sulfate (88.06% content interms of MnSO₄) dissolved in 16.843 kg of water and 2.570 l of an 8.5mol/l sodium hydroxide solution were concurrently added thereto over 16hours while the pH was kept at 6.5-7.5, to effect neutralization andoxidation and thus to make the manganese oxide seed grow. When the pHhad reached 6.4, a 20 l portion of the slurry was withdrawn and, whilean air-nitrogen gas mixture (1:1) was being blown into the slurry, anaqueous manganese sulfate solution and a sodium hydroxide solution wasconcurrently added thereto according to the same procedure as describedabove to effect further growth of the manganese oxide seed. When the pHhad reached 6.4, a 20 l portion of the slurry was again withdrawn andsuccessively, while an air-nitrogen gas mixture (1:1) was being blowninto the slurry at a rate of 5 l/min, an aqueous solution of 1.872 kg ofmanganese sulfate (88.06% content in terms of MnSO₄) dissolved in 8.422kg of water and 1.285 l of an 8.5 mol/l sodium hydroxide solution wereconcurrently added thereto over 8 hours while the pH was kept at6.5-7.5, to effect further growth of manganese oxide seed. When the pHhad reached 6.4, the growth reaction was finished, and the solid in theslurry was collected by filtration and washed with water to obtainmanganese oxide. The concentration of manganese ions remaining in thesolution at the completion of reaction was 30 g/l. (Sample b)

[0052] 4. Synthesis of Proton-Substituted Manganese Oxide

[0053] A slurry of manganese oxide (700 g in terms of Mn) dispersed inwater was placed in a stainless steel reaction vessel and warmed to 60°C. Into the slurry was added with stirring 2.039 l of 1 mol/l sulfuricacid over 1 hour; thereafter the resulting mixture was allowed to reactfor 2 hours, then filtered and washed with water to obtainproton-substituted manganese oxide.

[0054] 5. Synthesis of Lithium Manganate Precursor

[0055] To a slurry of the proton-substituted manganese oxide (500 g interms of Mn) dispersed in water was added 5.429 mols of lithiumhydroxide monohydrate and dissolved therein, then water was added to theslurry to make a volume of 1.25 l, and the resulting slurry was placedin a glass reaction vessel. Air was blown into the slurry at a rate of 2l/min and, with stirring, the slurry was heated to 90° C. and allowed toreact for 5 hours. Then the reaction mixture was transferred to anautoclave and subjected to a hydrothermal treatment at 130° C. for 3hours. The hydrothermally treated slurry was again placed in a glassreaction vessel and, while air was being blown thereinto at a rate of 2l/min to effect stirring, was further allowed to react at 90° C. for 1hour. After the reaction, the slurry was cooled to a temperature of 60°C., then filtered and washed with 2 l of a 0.1 mol/l lithium hydroxidesolution to obtain a lithium manganate precursor.

[0056] 6. Fireing of Lithium Manganate Precursor

[0057] Lithium manganate was obtained according to the same method as inExample 1.

[0058] 7. Densification of Lithium Manganate

[0059] The lithium manganate was subjected to densification according tothe same method as in Example 1. (Sample B)

EXAMPLE 3

[0060] 1. Synthesis of Manganese Hydroxide

[0061] A solution of 3.744 kg of manganese sulfate (88.06% content interms of MnSO₄) dissolved in 16.84 kg of water was placed in a stainlesssteel reaction vessel. Nitrogen gas was blown into the solution at arate of 5 l/min, the solution was heated to 70° C., and 2.57 l of an 8.5mol/l sodium hydroxide solution was added thereto with stirring over 1hour to effect neutralization. Thereafter the reaction mixture was agedfor 2 hours to obtain a solution containing manganese hydroxide. Theconcentration of manganese ions remaining in the solution afterneutralization was 30 g/l.

[0062] 2. Synthesis of Manganese Oxide Seed

[0063] A manganese oxide seed was obtained according to the same methodas in Example 1.

[0064] 3. Growth of Manganese Oxide Seed

[0065] The solution containing the manganese oxide seed obtained abovewas heated to 70° C., then, with stirring, an air-nitrogen (1:1) gasmixture was blown thereinto at a rate of 5 l/min, an aqueous solution of3.744 kg of manganese sulfate (88.06% content in terms of MnSO₄)dissolved in 16.843 kg of water and 2.57 l of an 8.5 mol/l sodiumhydroxide solution was added over 16 hours, to effect neutralization andoxidation, and thus to make the manganese oxide seed grow. When the pHhad reached 6.4, a 20 l portion of the slurry was withdrawn. While anair-nitrogen (1:1) gas mixture was being blown into the slurry at a rateof 5 l/min, an aqueous manganese sulfate solution and a sodium hydroxidesolution were added thereto according to the same procedure as describedabove, to effect further growth of manganese oxide seed. When the pH hadreached 6.4, a 20 l portion of the slurry was again withdrawn and, whilesuccessively an air-nitrogen (1:1) gas mixture was being blown into theslurry at a rate of 5 l/min, manganese sulfate and sodium hydroxide wereadded again according to the same procedure as described above, toeffect further growth of manganese oxide seed. When the pH had reached6.4 the growth reaction was finished, and the slurry was filtered andwashed with water to obtain manganese oxide. The concentration ofmanganese ions remaining in the solution at the completion of reactionwas 30 g/l. (Sample c)

[0066] 4. Synthesis of Proton-Substituted Manganese Oxide

[0067] Proton-substituted manganese oxide was obtained according to thesame method as in Example 2.

[0068] 5. Synthesis of Lithium Manganate Precursor

[0069] The proton-substituted manganese oxide (500 g in terms of Mn) wasdispersed in water, thereto was added 5.373 mols of lithium hydroxidemonohydrate and dissolved therein, then water was added thereto to makea volume of 1.111 l, and the resulting slurry was placed in a glassreaction vessel. Air was blown thereinto at a rate of 2 l/min, and theslurry was heated to 90° C. with stirring, then allowed to react for 5hours, thereafter transferred to an autoclave, and subjected to ahydrothermal treatment at 130° C. for 3 hours. The hydrothermallytreated slurry was placed again in a glass reaction vessel, air wasblown thereinto at a rate of 2 l/min, and the reaction was continuedwith stirring at 90° C. for 1 hour. After the reaction, the reactionmixture was cooled to 60° C., then filtered and washed with 2 l of a 0.1mol/l lithium hydroxide solution to obtain a lithium manganateprecursor.

[0070] 6. Fireing of Lithium Manganate Precursor

[0071] Lithium manganate was obtained according to the same method as inExample 1.

[0072] 7. Densification of Lithium Manganate

[0073] Densification was conducted according to the same method as inExample 1. (Sample C)

EXAMPLE 4

[0074] 1. Synthesis of Manganese Hydroxide

[0075] Manganese hydroxide was obtained according to the same method asin Example 3.

[0076] 2. Synthesis of Manganese Oxide Seed

[0077] A manganese oxide seed was obtained according to the same methodas in Example 1.

[0078] 3. Growth of Manganese Oxide Seed

[0079] To the solution containing the manganese oxide seed obtainedabove was added 40.260 kg of water while nitrogen gas was being blownthereinto at a rate of 5 l/min, further 11.231 kg of manganese sulfate(88.06% content in terms of MnSO₄) was added, and the resulting mixturewas stirred to dissolve the manganese sulfate. The resulting solutionwas heated to 70° C., then, with stirring, nitrogen gas was changed toan air-nitrogen (1:1) gas mixture and the gas mixture was blownthereinto at a rate of 5 l/min. Successively, 17.99 l of an 8.5 mol/lsodium hydroxide solution was added over 64 hours to effectneutralization and oxidation and thus to make the manganese oxide seedgrow. When the pH had reached 8.5 the growth reaction was finished, andthe reaction mixture was filtered and washed with water to obtainmanganese oxide. The concentration of manganese ions remaining in thesolution at the completion of reaction was 0 g/l. (Sample d)

[0080] 4. Synthesis of Proton-Substituted Manganese Oxide

[0081] Proton-substituted manganese oxide was obtained according to thesame method as in Example 1.

[0082] 5. Synthesis of Lithium Manganate Precursor

[0083] The proton-substituted manganese oxide (500 g in terms of Mn) wasdispersed in water, 5.373 mols of lithium hydroxide monohydrate wasadded thereto and dissolved therein, then water was added thereto tomake a volume of 1.111 l, and the resulting mixture was placed in aglass reaction vessel. Air was blown thereinto at a rate of 2 l/min and,with stirring, the reaction mixture was heated to 90° C. and allowed toreact for 15 hours. After the reaction, the reaction mixture was cooledto 60° C., then filtered, and washed with 2 l of a 0.1 mol/l lithiumhydroxide solution to obtain a lithium manganate precursor.

[0084] 6. Fireing of Lithium Manganate Precursor

[0085] Lithium manganate was obtained according to the same method as inExample 1.

[0086] 7. Crushing Treatment of Lithium Manganate

[0087] One hundred grams of lithium manganate after burning was crushedin an agate mortar. (Sample D)

COMPARATIVE EXAMPLE 1

[0088] 1. Synthesis of Manganese Hydroxide

[0089] A solution of 2.397 kg of manganese sulfate (88.06% content interms of MnSO₄) dissolved in 17.8 kg of water was placed in a stainlesssteel reaction vessel. Nitrogen gas was blown into the solution at arate of 5 l/min, the solution was heated to 70° C., and 6.825 l of a 4mol/l sodium hydroxide solution was added thereto with stirring over 1hour to effect neutralization and thus to obtain manganese hydroxide.The concentration of manganese ions remaining in the solution afterneutralization was 0 g/l.

[0090] 2. Synthesis of Manganese Oxide

[0091] While the solution containing manganese hydroxide obtained abovewas being stirred, air was blown into the solution at a rate of 5 l/minto effect oxidation at a temperature of 70° C. and, when the pH hadreached 7.0, air was changed to nitrogen to finish the oxidation.Thereafter the reaction mixture was filtered and washed with water toobtain manganese oxide. (Sample e)

[0092] 3. Acid Treatment of Proton-Substituted Manganese Oxide

[0093] Proton-substituted manganese oxide was obtained according to thesame method as in Example 1.

[0094] 4. Synthesis of Lithium Manganate Precursor

[0095] To a slurry of the proton-substituted manganese oxide (500 g interms of Mn) dispersed in water was added 5.675 mols of lithiumhydroxide monohydrate and dissolved therein, then water was added to theslurry to make a volume of 3.846 l, and the slurry was placed in a glassreaction vessel. Air was blown in at a rate of 3 l/min and, withstirring, the slurry was heated to 90° C. and allowed to react for 10hours. After the reaction, the slurry was cooled to 60° C., thenfiltered and washed with 2 l of a 0.1 mol/l lithium hydroxide solutionto obtain a lithium manganate precursor.

[0096] 5. Fireing of Lithium Manganate Precursor

[0097] Lithium manganate was obtained according to the same method as inExample 1 except that the fireing of lithium manganate precursor wasconducted at 800° C. for 3 hours.

[0098] 6. Densification of Lithium Manganate

[0099] Densification was conducted according to the same method as inExample 1 (Sample E)

COMPARATIVE EXAMPLE 2

[0100] Lithium manganate was obtained according to the same method as inComparative Example 1 except that the fireing of lithium manganateprecursor was conducted at 850° C. for 3 hours. (Sample F)

COMPARATIVE EXAMPLE 3

[0101] Lithium manganate was obtained according to the same method as inComparative Example 1 except that the fireing of lithium manganateprecursor was conducted at 900° C. for 3 hours. (Sample G)

[0102] Evaluation 1

[0103] The manganese oxide seed, or manganese oxide (samples a-e), andlithium manganate (samples A-G) obtained in Examples 1-4 and ComparativeExamples 1-3 were determined for their specific surface areas by using aspecific surface area measuring apparatus (Monosorb, a trade name, mfd.by Yuasa Ionics Inc.) and according to the BET method.

[0104] Evaluation 2

[0105] The aqueous slurries of the manganese oxide seed, or manganeseoxide (samples a-e), and lithium manganate (samples A-G) obtained inExamples 1-4 and Comparative Examples 1-3 were sufficiently dispersedultrasonically so as to attain transmittance by laser light of 85±1%,and then determined for their average particle diameters by using aparticle size distribution measuring apparatus of laserdiffraction-scattering system (LA-90, a trade name, mfd. by HORIBA,Ltd.) by volume base.

[0106] Evaluation 3

[0107] Each 50 g of the lithium manganate (samples A-G) obtained inExamples 1-4 and Comparative Examples 1-3 was placed in a 100-mlmeasuring cylinder and tapped 100 times to determine the tap density.

[0108] Evaluation 4

[0109] Lithium secondary batteries wherein lithium manganate (samplesA-G) obtained in Examples 1-4 and Comparative Examples 1-3 was used asthe positive electrode active material were evaluated for their chargeand discharge characteristics and cycle characteristics. The batterieswere of a three-electrode system and were subjected to repeated chargeand discharge. The shape of the batteries and the determinationconditions are described below.

[0110] Each of the above-mentioned samples was mixed with graphitepowder as a conductive material and polytetrafluoroethylene resin as abinder in a weight ratio of 3:2:1, then kneaded together in a mortar,and formed into a disk 14 mm in diameter, to obtain a pellet. The weightof the pellet was 50 mg. The pellet was held between metallic titaniummeshes and pressed at a pressure of 14.7 MPa, to be used as a positiveelectrode.

[0111] On the other hand, a metallic lithium sheet 0.5 mm in thicknesswas formed into a disk 14 mm in diameter, which was held betweenmetallic nickel meshes and pressure-bonded to be used as a negativeelectrode. Separately, metallic lithium foil 0.1 mm in thickness waswound round a metallic nickel wire so as to be approximately the size ofrice grain, to be used as a reference electrode. As a nonaqueouselectrolyte was used a solvent mixture of 1,2-dimethoxyethane andpropylene carbonate (1:1 by volume) containing lithium perchloratedissolved therein in a concentration of 1 mol/l. The electrodes werearranged in the order of positive electrode, reference electrode andnegative electrode, and porous polypropylene film was placedtherebetween as a separator.

[0112] The charge and discharge capacity was determined at constantcurrent with voltage set in the range of 4.3 V-3.5 V and charge anddischarge current set at 0.26 mA (about 1 cycle/day). As to cyclecharacteristics, the charge and discharge capacities at the second cycleand the 11th cycle were determined, and the cycle characteristic wasexpressed by the capacity retention rate in charge or discharge (%)[{1—(charge or discharge capacity at the second cycle—charge ordischarge capacity at the 11th cycle)/charge or discharge capacity atthe second cycle}×100].

[0113] Table 1 shows the specific surface areas and average particlediameters of Samples a-e, Table 2 the specific surface areas, averageparticle diameters and tap densities of Samples A-G, Table 3 the initialcharge and discharge characteristics and cycle characteristics of theSamples A-G. TABLE 1 Specific Average particle surface area diameterSample (m²/g) (μm) Example 1 a 1.57 4.44 Example 2 b 1.59 4.81 Example 3c 1.05 5.78 Example 4 d 1.13 5.66 Comparative e 21.50 0.51 Example 1

[0114] TABLE 2 Specific Average surface particle Tap area diameterdensity Sample (m²/g) (μm) (g/cm³) Example 1 A 2.84 3.00 1.85 Example 2B 2.75 3.46 1.98 Example 3 C 2.37 4.72 2.00 Example 4 D 1.05 6.02 2.02Comparative E 3.70 1.80 1.61 Example 1 Comparative F 3.00 4.01 1.82Example 2 Comparative G 0.60 7.31 2.00 Example 3

[0115] TABLE 3 Cycle characteristic Initial charge discharge capacitycharacteristics retention rate Charge Discharge (%) (mAh/ capacitycapacity Effi- Dis- Sample g) (mAh/g) ciency (%) Charge charge Example 1A 114.4 112.8 98.6 96.0 95.9 Example 2 B 116.2 114.9 98.9 97.9 96.8Example 3 C 119.0 117.0 98.4 96.6 97.2 Example 4 D 116.1 114.6 98.7 97.297.5 Compara- F 117.2 116.9 98.9 93.2 92.8 tive Example 1 Compara- F105.4 103.2 97.9 88.2 87.6 tive Example 2 Compara- G 100.2 98.8 98.685.0 82.1 tive Example 3

INDUSTRIAL APPLICABILITY

[0116] In the process of this invention, at a step precedent to theproduction of lithium manganate, manganese oxide seed are prepared andmade to grow in a solution to form manganese oxide having large particlediameters. Since intended large particles can be obtained at this step,the particle diameters and particle shapes of the final product can bemade more uniform as compared in the prior method wherein largeparticles are formed by sintering lithium manganate; moreover theparticle diameters and particle shapes can be easily controlled byproperly setting the conditions of seed growth reaction. Furthermore,since the manganese oxide which has undergone particle growth and alithium compound are reacted with each other in a solution or,alternatively, proton-substituted manganese oxide obtained byacid-treating the above-mentioned manganese oxide is reacted with alithium compound, lithium manganate having excellent crystallinity anduniform composition can be obtained. The lithium manganate obtained bythe process of this invention has large particle diameters as describedabove, shows a high packing density and can provide a lithium batterywith a high energy density by using it as the positive electrode activematerial.

1. A process for producing lithium manganate which comprises (1) thestep of reacting a manganese compound with a basic compound to obtainmanganese hydroxide, (2) the step of oxidizing the manganese hydroxideto obtain manganese oxide seed, (3) the step of conducting oxidationwhile reacting a manganese compound with a basic compound in thepresence of the manganese oxide seed to cause the particle growth of themanganese oxide seed, (4) the step of reacting the manganese oxide whichhas undergone particle growth with a lithium compound, or the step oftreating the manganese oxide with an acid to obtain proton-substitutedmanganese oxide and then mixing the proton-substituted manganese oxidewith a lithium compound or reacting it with a lithium compound and (5)the step of fireing a reaction product or a mixture obtained above withheating.
 2. The process for producing lithium manganate according toclaim 1 wherein the lithium manganate has an average particle diameterof 0.4-50 μm.
 3. The process for producing lithium manganate accordingto claim 1 wherein in reacting the manganese compound with the basiccompound in the first step, partial neutralization is conducted.
 4. Theprocess for producing lithium manganate according to claim 3 wherein thepartial neutralization is conducted such that the concentration ofmanganese ions remaining in the solution after the partialneutralization becomes 5-60 g/l.
 5. The process for producing lithiummanganate according to claim 1 wherein, in the second step, oxidation isconducted with an oxidizing gas or an oxidizing agent.
 6. The processfor producing lithium manganate according to claim 5 wherein theoxidizing gas is air, oxygen or ozone and the oxidizing agent is anaqueous hydrogen peroxide solution or a peroxodisulfate.
 7. The processfor producing lithium manganate according to claim 1 wherein, in thethird step, partial neutralization is conducted at the time of reactingthe manganese compound with the basic compound.
 8. The process forproducing lithium manganate according to claim 7 wherein the partialneutralization is conducted such that the concentration of manganeseions remaining in the solution after the partial neutralization becomes5-60 g/l.
 9. The process according to claim 1 wherein, in the thirdstep, a solution of the manganese compound and a solution of the basiccompound are concurrently added to a solution containing the manganeseoxide seed.
 10. The process for producing lithium manganate according toclaim 1 wherein, in the third step, oxidation is conducted with anoxidizing gas or an oxidizing agent.
 11. The process for producinglithium manganate according to claim 10 wherein the oxidizing gas isair, oxygen or ozone, and the oxidizing agent is an aqueous hydrogenperoxide solution or a peroxodisulfate.
 12. The process for producinglithium manganate according to claim 1 wherein, in the fourth step, theacid used for treating the manganese oxide is at least one memberselected from the group consisting of hydrochloric acid, sulfuric acid,nitric acid and hydrofluoric acid.
 13. The process for producing lithiummanganate according to claim 1 wherein, in the fourth step, the reactionof the manganese oxide or the proton-substituted manganese oxide withthe lithium compound is conducted by means of hydrothermal treatment.14. The process for producing lithium manganate according to claim 1wherein, in the fourth step, the reaction of the manganese oxide or theproton substituted manganese oxide with the lithium compound isconducted while an oxidizing gas or oxidizing agent is being supplied tothe reaction system.
 15. The process for producing lithium manganateaccording to claim 14 wherein the oxidizing gas is air, oxygen or ozone,and the oxidizing agent is an aqueous hydrogen peroxide solution or aperoxodisulfate.
 16. The process for producing lithium manganateaccording to claim 1 wherein, in the fifth step, a product obtained byfireing is subjected to densification.
 17. The process for producinglithium manganate according to claim 1 wherein the manganese compound isat least one member selected from the group consisting of manganesesulfate, manganese chloride and manganese nitrate.
 18. The process forproducing lithium manganate according to claim 1 wherein the lithiumcompound is at least one member selected from the group consisting oflithium hydroxide, lithium carbonate, lithium hydrogen carbonate,lithium chloride and lithium sulfate.
 19. A lithium battery which usesthe lithium manganate obtained by the process according to claim 1 or 2as a positive electrode active material.